The present invention relates generally to x-ray imaging systems and more particularly, to exposure management in an x-ray imaging system.
Traditionally, in x-ray imaging systems, exposure management is handled by an automatic exposure control subsystem. These subsystems typically utilize a small number of predefined, fixed sensor areas that terminate the exposure when a predefined limit has been reached. The sensors areas are x-ray sensitive and usually include an ion chamber. The automatic exposure control subsystem requires dedicated hardware and calibration that is costly and difficult to maintain.
A further disadvantage of these subsystems with fixed sensors is that minimal, if any, adjustments can be made for various patient sizes, anatomical views/orientations, pathologies, or system geometries. Consequently, an optimal exposure level is not always reached, especially in cases where patient characteristics are atypical. The patient must be positioned such that the anatomy of interest is superimposed on the ion-chamber location. This is sometimes difficult and can lead to variable exposures from patient to patient and from exposure to exposure. While some portions of the image may be properly exposed, other portions are improperly exposed with some regions underexposed and other regions overexposed.
Improperly exposed images require a retake of the x-ray, which results in increased radiation dose to a patient, increased patient discomfort, increased cost to the hospital, and decreased hospital productivity.
It is therefore one object of the invention to predict the appropriate exposure technique parameters for an x-ray image. It is another object of the present invention to predict optimal exposure technique parameters for an x-ray image. It is still a further object of the present invention to use a low-dose preshot image and virtual ion chambers to select and control the diagnostic exposure to be imaged in a subsequent shot.
In one aspect of the invention, an automatic exposure control mechanism is provided that uses a low-dose preshot image and virtual ion chambers to select and control the diagnostic exposure to be imaged in a second shot directed to an identified region of interest. The virtual ion chambers can be configured for various patient sizes, anatomical views/orientations, pathologies, and system geometries.
According to the present invention, a virtual ion chamber or soft sensor area is defined by an overlay having divisions therein whereby all, or part, of the overlay can be selected to define a region of interest in a digital x-ray image. The region of interest may be defined by any configuration, or combination, of shapes and sizes within a given x-ray image. In one embodiment of the present invention, the region of interest is defined using a matrix of Nxc3x97M simple geometric shapes.
Other objects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.